Implantable defibrillators are well known. A critical element of these devices is the lead system which provides electrodes to sense the electrical activity of the heart and to deliver an appropriate electrical stimulation in the event the device detects an abnormal rhythm. The most popular type of lead system for use with these devices is a transvenous lead system in which the electrodes are carried on the distal end of an elongated catheter that is inserted into a vein near the heart and then moved into position, typically in either the right atrium or right ventricle, after which the proximal end is connected to the device in order to provide an electrical connection between the electrodes and the device.
In a typical configuration for an implantable defibrillator, for example, the transvenous lead may have a pacing tip electrode at the very most distal end which is positioned in the apex of the right ventricle. A defibrillation coil electrode spaced back from the pacing tip is positioned to lie along the bottom and side of the right ventricle. Sensing and pacing can be accomplished in a unipolar manner between the tip electrode and the defibrillation electrode. Alternatively, sensing and pacing can be accomplished in a bipolar manner by providing an additional ring electrode spaced adjacent from the tip electrode. Defibrillation can be accomplished between the defibrillation electrode in the right ventricle and the housing of the device located in the pectoral region or a subcutaneous electrode located on the left side of the body. For dual chamber applications in which pacing and sensing is needed in both the right atrium and the right ventricle, an additional catheter carrying a pair of pace/sense electrodes is typically located in the right atrium.
Although the typical transvenous lead system offers many advantages in terms of ease of implantation and good sensing, it is well known that a transvenous defibrillation lead system is less efficient than a lead system using epicardial patches. However, epicardial patch electrodes have the disadvantage of requiring open heart surgery for implantation. Because the effectiveness of both the electrical sensing and stimulation of these devices depends upon the characteristics and performance of the lead system, many alternative lead systems for implantable defibrillators have been proposed.
In U.S. Pat. Nos. 4,932,407 and 5,014,696, a lead system is described in which a transvenous lead is positioned in the coronary sinus vein, a vein which is accessed through the right atrium. To accomplish defibrillation, these patents teach the delivery of a defibrillation countershock between defibrillation electrodes positioned in the right ventricle and the coronary sinus plus a subcutaneous electrode. In U.S. Pat. No. 5,099,838, a similar lead system is described in which delivery of a defibrillation countershock is taught between a coronary sinus electrode and a subcutaneous electrode. Subsequent research by the inventor of the '838 patent suggested that the addition of a coronary sinus electrode did not increase the overall effectiveness of defibrillation as compared to a conventional lead system. Bardy, "A Prospective Randomized Comparison in Humans of the Unipolar Pectoral Defibrillation System with one Incorporating an Additional Electrode in the Coronary Sinus," Circulation Vol. 88, No. 4, Part 2, Oct. 1993, 1155.
In U.S. Pat. Nos. 5,314,430, 5,348,021, 5,350,404, 5,433,729 and 5,476,498, lead systems for an atrial defibrillator are described in which electrodes placed in the coronary sinus and its extension, which wraps around to the front of the heart and is known as the great vein, are used in conjunction with electrodes in the right ventricle or super vena cava, as well as subcutaneous electrodes, to defibrillate the atria. Because these lead systems are used primarily for defibrillating the atria or upper chambers of the heart, the coronary sinus vein and the great vein which wrap around the atria provide an ideal location for an atrial defibrillation lead system. Unfortunately, initial clinical experiences with these types of atrial defibrillation lead systems have shown that, while these lead systems are effective for atrial defibrillation, the pain threshold associated with the atrial defibrillation using these lead systems is often not tolerable. Lau et al., "Initial Clinical Experience with an Implantable Human Atrial Defibrillator," PACE, Vol. 20, Jan. 1997, Part II, pp 220-25; Heisel et al., "Low-Energy Transvenous Cardioversion of Atrial Fibrillation Using a Single Atrial Lead System," Journal of Cardiovascular Electrophysiology, Vol. 8, No. 6, June 1997, pp. 607-14; and Lok et al., "Electrode Sites for Transvenous Atrial Fibrillation," JACC, Vol. 30, No. 5, Nov. 1997, 1324-30.
In U.S. Pat. Nos. 5,755,765 and 5,755,766, a transvenous cardiac lead is described for use in the coronary sinus and great vein. This lead is provided with an internal lumen that is open at the distal end to allow a secondary electrode to be inserted through the lumen and extended beyond the distal end of the lead. The lead is also shown as having a reduced diameter at its most distal section. Disclosed techniques for implanting the lead in the coronary sinus include temporarily fixing the lead within a guiding catheter or using a guide wire in the open lumen. While this lead system is primarily intended for use in as part of a technique to pace both the right and left ventricles in an effort to improve the hemodynamic efficiency of the heart, it is also disclosed that the electrodes on this lead system may be used for defibrillation. Other pacing leads which are adapted for implantation in the coronary sinus and great vein are described in U.S. Pat. Nos. 5,545,204 and 5,755,761. As with the lead systems for atrial defibrillators, the objective of these pacing leads is to wrap around to the anterior or front side of the heart with the distal end of the lead.
In PCT Appl. No. WO 98/13102 a pacing lead is disclosed as part of a combination pacing/defibrillation system in which the pacing lead is introduced into the coronary sinus, then into the great cardiac vein, and then into the ascending limb of either the anterior cardiac vein or the posterior cardiac vein. The objective of the placement of this pacing lead is to locate the electrode as close as possible to the apex of the left ventricle. In PCT Appl. No. WO 98/14241 a pacing lead is disclosed for providing multisite anodal stimulation to improve cardiac output. One embodiment of the lead is lodged deep in the great rein and coronary sinus and optionally extending toward the ventricular apex into the great or middle cardiac vein.
Most transvenous defibrillation lead systems are constructed of a catheter tube having one or more lumens through which conductor wires are strung and then attached to ring or coil electrodes located near the distal end of the tube. Several alternative constructions for transvenous lead systems also have been proposed. In U.S. Pat. No. 5,304,218, a transvenous lead system is described in which the catheter lead is placed in position using an over-the-wire guide. In U.S. Pat. Nos. 5,282,845 and 5,551,426, multiple precurved electrode segments are deployed from the distal end of the catheter tube within a chamber of the heart in an effort to create a single common electrode having a larger effective surface area. Similarly, in U.S. Pat. Nos. 5,203,348 and 5,360,442, a single subcutaneous electrode is provided with multiple finger elements that are electrically connected in common to emulate a single electrode having a larger effective surface area. In U.S. Pat. No. 5,531,779, a stent type of defibrillation electrode is disclosed which expands to conform to the interior of a vein.
While numerous transvenous defibrillation lead systems have been proposed, there are certain patients for which the defibrillation lead system is not as efficient as would be desired. As a result, the defibrillation threshold for successfully defibrillating these patients is larger than may be acceptable in terms of an appropriate safety margin for a given implantable defibrillator. It would be advantageous to provide a transvenous defibrillation lead system which had improved characteristics and performance over existing transvenous defibrillation systems, particularly for patient who exhibit high defibrillation thresholds with existing transvenous defibrillation lead systems.